The case should help with the year, the last three digits are as follows
1963 case number 704 corvettes only as far as I have seen
1963 and very early 64 had case number 606 very rare case used in corvettes, never seen in a GTO by me, but 64 corvettes got them
64 case number 325 with pat pending very early cars before Jan 64
64-65 case number 325 with pat number
66-67 case number 010
68-69 case number 660
70-74 case number 661
always pop covers as even replacement Muncie input gears had no grooves
Muncie close ratio heavy duty (m22) had no grooves on production line, but no heavy duty 63-65 m22 build (save for a very few rare road race corvette) many 63-65 have no grooves, this is why you have to pop side cover
Muncie close ratio (m21) had one groove
Muncie wide ratio (m20) had two grooves 66 and newer
M20 and M21 are not exclusive to the Muncie as this is the GM RPO code and Saginaw used this as well (Camaros and non SS Chevelles and other GM's that used both cases use The M20 wide ratio and M21 close ratio for Muncie and Saginaw cases)
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